How exactly do you think it is harming them? (not teenagers and so-on), curious of your take. Sometimes treatments are useful even if you can't get out of it. AI companions will never disappear, that's sure, so it's not like giving something temporarily and then removing it.
I don't really use any form of AI as companion, but I do use it for introspection, I wouldn't say it's harmful, I think it depends on the depth of it.
I think that computers can't be a meaningful companion, because they aren't actually sentient. They can't truly sympathize with you, nudge you to grow when needed, and so on. Moreover, I think that treating a machine as a friend (or a lover) is going to stunt one's ability to connect with actual humans.
It's kind of like (though not exactly the same as) guys who watch crazy amounts of porn and jerk off all the time. Those guys have reported that when they are with a real woman, they sometimes can't even get hard (because a real person can't match the stimulation of the artificial thing designed to be as stimulating as possible), or can't have an orgasm (because a vagina isn't as tight as the grip they use when jerking it). In those cases, the outlet those guys were using to fill that need has ruined their ability to enjoy the healthier alternative. I believe that AI "companionship" is going to prove to have a similar effect.